The present invention relates to a linear power amplification method and a linear power amplifier for use in a radio communication transmitter, for instance.
One of nonlinear distortion compensating schemes for microwave power amplifiers is a predistortion scheme using digital signal processing (hereinafter referred to as a digital distortion scheme) (for instance, H. Girard and K. Feher, “A new baseband linearizer for more efficient utilization of earth station amplifiers used for QPSK transmission,” IEEE J. on Selected Areas in Commun. VOL. SAC-1, NO. 1, January 1983). A feature of the digital predistortion scheme resides in obviating the necessity of using complex analog circuitry by implementing the operation of a predistorter through digital signal processing. Conventional linear amplifiers are formed primarily by analog circuits such as a feedforward amplifier and a negative feedback amplifier. The predistorter is also implemented in analog form (for example, Nojima, Okamoto, and Ohyama, “Predistortion Nonlinear Compensator for Microwave SSB-AM System,” Transactions of IEICE of Japan, '84/1 VOL. J67-B NO. 1, pp. 78–85).
Linearization technology using these analog circuits, however, generally calls for sophisticated adjustment techniques. Furthermore, miniaturization and economization of transmitters including a modulation circuit require simple configuration of analog circuits. In this respect, the digital predistorter, which implements linearization through digital signal processing, is advantageous over the conventional predistorter that employs analog circuits. Moreover, an amplifier using the predistorter is capable of achieving high efficiency amplification since it has no analog circuit for linearization, such as an auxiliary amplifier used in the feedforward amplifier.
A known configuration of the digital predistorter uses a lookup table for pre-linearization of nonlinear characteristics of amplifiers (for example, L. Sundstrom, IEEE, M. Faulkner, and M. Johansson, “Quantization analysis and design of a digital predistortion linearizer for RF power amplifiers,” IEEE Trans. Vech. Tech., VOL. 45, NO. 4, pp707–719, November 1996). The digital predistorter using the lookup table updates set values in the lookup table by feeding back amplifier output signals so that distortion components go down below a preset value. It is known in the art that distortions can thus be compensated by digital signal processing and that the compensated amount of distortion is approximately 15 dB or below (Y. Oishi, N. Tozawa, and H. Suzuki, “Highly Efficient Power Amplifier for IMT-2000 BTS Equipment,” FUJITSU Sci. Tech. J., 38, 2, p. 201–208, December 2002). To maximize the efficiency of amplification by the power amplifier, it is necessary to compress the output backoff of the amplifier by increasing the amount of distortion to be compensated for. FIG. 1 shows the relationship between the output backoff from a 1-dB gain compression point and the efficiency of amplification. The condition for review is an ideal class “B” bias. From FIG. 1, it will be seen that greater amplification efficiency can be achieved by increasing the amount of distortion to be compensated for to such an extent as to enable compression of the output backoff.
FIG. 2 shows the relationships between the distortion reduction and amplitude and phase deviations of a third-order distortion component. To achieve distortion compensation performance at least above 30 dB, a digital predistorter is needed which yields an amplitude deviation within ±0.2 dB and a phase deviation within ±2 deg. As will be seen from FIG. 2, the digital predistorter is required to attain predetermined amplitude and phase deviations in accordance with secular and temperature variations as well.
To realize distortion compensation (distortion improvement) in excess of a value attainable at present, the conventional lookup table type digital predistorter needs to be equipped, as will be understood from FIG. 3, with a high-precision lookup table for maintaining the distortion compensation at a high level. Further, it is necessary to provide a control route which, when a nonlinear characteristic of the power amplifier slightly changes with a temperature deviation or secular variation, monitors the amplifier output signal and corrects the lookup table accordingly.
On the digital predistorter using the lookup table, however, the relationships between distortion components and values set in the lookup table have not been clarified nor has been presented any concrete method for correcting a slight variation in the nonlinear characteristic of the amplifier that is caused by a secular or temperature change, for instance.
One approach to high-precision compensation for distortion components is a predistorter based on a power series model. Such a predistorter has been implemented so far using analog circuits, and its distortion improvement performance is above 30 dB (for instance, T. Nojima and T. Konno, “Cuber predistortion linearizer for relay equipment in 800 MHz band land mobile telephone system,” IEEE Trans. Vech. Tech., VOL. VT-34, NO.4, pp169–177, November 1985). It is known in the art that the power series model is one that models nonlinear characteristics of the amplifier with high precision (for example, Tri T. Ha, “Solid-State Microwave Amplifier Design,” Chapter 6, Krieger Publishing Company, 1991). With the distortion compensation scheme of the digital predistorter using the power series model, signals for correcting coefficients of respective orders need to be extracted from the amplifier output signal. In British Patent Application Publication GB2335812A there is described the extraction of such correction signals by removing distortion component of the fundamental wave and higher orders from the transmission signal. A scheme for more easy extraction of the correction signals of the power series model is to use two carriers of the same levels as pilot signals. (see the afore-mentioned document by T. Nojima and T. Konno).
There have been proposed improving the frequency dependence of the nonlinear characteristic of the power amplifier as well as compensation for its temperature dependence. With a view to implementing excellent compensation for distortion in a wideband signal by the conventional predistorter, Japanese Patent Application Publication No. 11-17462 proposes reduction of the path difference between the main signal path and the distorted signal path, and Japanese Patent Application Publication No. 7-7333 proposes the connection of a phase equalizer to the input signal line. The reason for using such schemes is to cause the distortion generated by the predistorter to vary with a fixed gain and in a fixed phase over a wide frequency band.
However, widening of the frequency band for amplification provides increased frequency deviation in the gain and phase characteristics of the power amplifier as shown in FIG. 3, for instance,—this exerts nonnegligible influence on signal amplification. On this account, only by fixedly varying the amplitude and phase of the distortion over the entire frequency band, it is impossible that the distortion by the predistorter remains over the entire frequency band at a level for canceling the distortion by the power amplifier and opposite thereto in phase. Accordingly, to implement high-precision distortion compensation, it is necessary that frequency dependent amplitude and phase characteristics of the distortion by the predistorter be varied in such a manner as to cancel frequency deviations of gain and phase characteristics of the power amplifier. Japanese Patent Application Publication No. 10-327209 proposes the use of an equalizer to vary the frequency-amplitude and frequency-phase characteristics of the distortion generated by the predistorter.
For example, in the conventional predistorter shown in Japanese Patent Application Publication No. 2002-64340, the output from an analog distorter is adjusted in amplitude and phase at the higher- and lower-frequency sides of the fundamental wave output signal independently of each other to impart frequency characteristics to the distortion for compensation. In Japanese Patent Application Publication No. 2002-57533 an amplitude-frequency characteristic adjusting circuit composed of a band-pass filter and a vector adjuster is connected to the output side of an analog distorter so that the distortion for compensation has a frequency characteristic.
In the case of extracting an intermodulation distortion component of the amplifier output by a narrow-band filter and correcting each order coefficient of the analog predistorter, the coefficient can easily be corrected in a sufficiently short time for the transmission signal in a pilot signal feedback route in the analog predistorter. In contrast to the analog predistorter, the lookup table type digital predistorter involves digitization of the pilot signal monitored from the amplifier output, giving rise to a problem of delay in the feedback route.
In the analog predistorter the pilot signal is generated by an analog oscillator, whereas in the digital predistorter the pilot signal needs to be generated in the base band through digital signal processing. No concrete techniques or schemes have been proposed so far for signal conversion of the pilot signal and the transmission signal in the digital predistorter and for their analog-to-digital conversion.
In other words, it is still unclear how to configure the digital predistorter that uses the pilot signal. There is a demand for a simple configuration of the digital predistorter that achieves a high degree of distortion compensation and always performs distortion compensation according to secular and temperature variations.
The scheme of varying the frequency characteristic of the distortion generated by the predistorter through use of an equalizer, described in the afore-mentioned Japanese Patent Application Publication No. 10-327209, is to make uniform the frequency characteristics of the feedback route that controls the predistorter. This scheme does not take into consideration the frequency deviations of the gain and phase characteristics in the power amplifier. Accordingly, there arises the necessity for a predistorter capable of adjusting the frequency-amplitude and frequency-phase characteristics of the distortion generated by the predistorter in such a manner as to cancel the frequency deviations of the gain and phase characteristics in he power amplifier.
When the input signal is one that has discrete spectra on the frequency axis as in the case of using two carriers of the same amplitude, it is effective to impart the frequency characteristic to the distortion component by adjusting its amplitude and phase on the higher- and lower-frequency sides of the fundamental wave signal as proposed in the afore-mentioned Japanese Patent Application Publication No. 2002-64340. With this method, however, when the input signal has a continuous spectrum on the frequency axis like a modulated wave signal, it is impossible to provide the distortion component with such frequency characteristics that it continuously varies on the frequency axis. In the afore-mentioned Japanese Patent Application Publication No. 2002-57533 many band-pass filters and vector adjusters need to be prepared for imparting frequency characteristics to high-order distortions for compensation, too. Besides, it is also still unclear how to implement the frequency characteristics of compensating distortions for canceling the frequency characteristics of distortion component generated by the power amplifier. The predistorters disclosed in the afore-mentioned Patent Application Publication Nos. 2002-64340 and 2002-57533 are predistorters formed by analog elements. In this instance, implementation of the frequency characteristics for the compensating distortions calls for taking into account the frequency characteristics of the entire transmission system including the distorter, the vector adjuster and so on, as well as the frequency characteristics of the power amplifier.